This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A065804 #5 Mar 30 2012 18:51:35 %S A065804 1,1,2,4,6,7,24,120,9,40320,11,3628800,13,479001600,15,87178291200,17, %T A065804 20922789888000,19,6402373705728000,21,2432902008176640000,23, %U A065804 1124000727777607680000,720,26,15511210043330985984000000,28 %N A065804 Earliest sequence with a(a(n))=n!. %H A065804 <a href="/index/Aa#aan">Index entries for sequences of the a(a(n)) = 2n family</a> %e A065804 a(3)=4 since any lower value would suggest a(a(3)) was not 3!=6. So a(4)=a(a(3))=3!=6. a(5)=7 since any lower value would prevent a(a(5))=5!. a(6)=a(3!)=a(a(a(3)))=a(a(4))=4!=24. a(7)=a(a(5))=5!=120. a(8)=9 since any lower value would prevent a(a(8))=8!. etc. %Y A065804 Cf. A002516, A038752, A054048, A054049, A054790, A054791. %K A065804 nonn %O A065804 0,3 %A A065804 _Henry Bottomley_, Dec 06 2001